alsa-mixer: Fix path subset detection

alsa-mixer: Fix path subset detection

The old logic assumed that if path A was a subset of path B, the
element list in B would have all elements of A in the beginning of
B's list, in the same order as A. This assumption was invalid, causing
some subset cases to not get detected. We need to search through the
full element list of B every time before we can conclude that B
doesn't have the element that we're inspecting.